But still: What can be done or used to have prettier soft soldered
joint?

Use as little as possible. TIX brand soft solder is a bit
harder/stronger than some of the others, and melts lower, so it may
let you use less. Plus, it tends to stay brighter than some others.
But it’s still a white soft solder.

Do the job some other way. Laser welders and PUK welders, for
example, can often directly weld things that one used to have to use
soft solders for. Even if you don’t have the equipment, some pieces
are worth jobbing out so someone who does have the equipment, in
order to avoid the unfortunate use of soft solders. Other options may
include cold joints, like rivets or screws, or the like. A few cases
might even be appropriate for some glues, which aren’t much better
than soft solders for strength, but at least, can sometimes be less
visually obvious.

And if you must use soft solder, it can often be gold plated
(brush platers are good for such spot plating. Not always a great
quality plate, but you usually only need a bit of coverage to mask
the obvious color difference. There used to be, and may still, be a
product specifically marketed to give a copper plate to soft solder
just by brushing on. You might even get the same thing just with
copper sulphate solution or well used (bluish color) pickle. Not
sure about that, but try it. The copper color isn’t a match for
yellow gold either, but is closer, and might be less obvious.

you could of course plate it with a pen and 22kt solution, or Hoover
and strong’s yellow silver solder is quite good for yellow gold up
to 18kt- and in standard (not their “royal” yellow) yellow in which
the fine silver content is at or about 0.213-0.100 grams per gram of
fine gold…anything higher karat or richer yellow (0.045 -to pure 24
kt) will be too pale

If you are using lead-free tin solder then the solder can be gold
plated. I use cyanide gold plating solutions and they plate on tin
but I don’t know if acid based gold plating will. It’s worth testing
on a sample and finding out.

I only use tin solder on a gold piece if the alternative is so
expensive or risky that with full disclosure of the consequences the
customer agrees to it.

The seam of tin can be almost invisible provided that a piece of
gold in the form of a base plate or a patch is attached with the tin
solder and then cleaned up and polished. I mark on the back of the
piece “tin sol” to inform future workers (could be me) that there is
tin on the piece and thus they know to look for it, and not try any
gold soldering at a later date without first removing the tin.

I just finished a small repair where I had to soft solder on
(very) yellow gold. The visual contrast of the silver tin solder
and the gold is quite gross, fortunately it's the back of the
piece.

We were taught never ever to use soft/ tin/ lead based solder on
silver or gold, because if the piece is ever reheated the lead in the
soft solder eats through the precious metal. OK, some newer solders
may not contain lead any more, but the soft solder will preclude any
further heating because of its low melting temp.

But still: What can be done or used to have prettier soft soldered
joint?

Glue! 21st century aviation technology has produced some wonderful
adhesives.

When I’m forced to soft solder, (which I’m not anymore, now that I
have the laser) the trick is to find a way to hide the solder. For
example take an earring post. Just soft soldering a plain post will
look ugly and have no strength besides. Use a pearl cup minus the
peg. Under fill the cup with solder because it will expand. Then
treat it like sweat soldering. Clean up excess. For pin findings,
make a bar to serve the same purpose. the general principle can work
for many different circumstances.

I’ve never had luck trying to gold plate soft solder. A real old
trick was allegedly to apply some iodine but that never worked for me
either.

Have to strongly agree with Jeff on this even though I don’t work
with much gold only silver I just had almost a pound of silver ruined
by some sort of solder that did not belong there. Now how do I get it
out after I made sheet and wire with it. Every thing I make turns the
wearer skin. But there is not enough to attract a magnet.? Could use
a few words right now but think it might get edit out any way.

I’m in the never do it camp. So I won’t tell you how to put it on or
how to gold plate it, but I can tell you how to take it off.

I have had to remove soft/lead solder from many gold items in the
past so that I could repair them with gold. A good method is to soak
the item in muriatic acid overnight. The lead solder will be dark a
brittle the next morning and will flake and polish right off,
leaving no residue. It works every time. As with most things…do it
with good ventilation, the lead solder bubbles while dissolving.

Actually, the recipe I had was to use enough muriatic acid to cover
the piece, add a pinch of red rouge and heat up the solution until
warm. Then soak the item as long as needed. I skipped the rouge and
the heat and it has worked just fine.

I just had almost a pound of silver ruined by some sort of solder
that did not belong there. Now how do I get it out after I made
sheet and wire with it. Every thing I make turns the wearer skin.
But there is not enough to attract a magnet.

If you actually melted it into ingots and made sheet and wire, and
it was contaminated with junk that shouldn’t be there, the metal
needs to be refined to clean it up. Send it in to your favorite
refiner, perhaps in trade for new sheet or wire, or less costly
casting grain if you prefer to make your own. By the way, tin/lead or
related solders won’t be affected by a magnet. Nickle or iron are
magnetic. Tin and lead and similar metals in those soft solders, are
not. Be sure, though, that the problem is with your metal before you
take drastic action. Is the discoloration only with one customer? If
so, try them with some commercially alloyed silver to compare. There
are some people, as with any metals, who will have a problem with
silver even when it’s not contaminated. As a general rule, I’d have
expected you to have a clue that there was a problem before you got
to the finished stage, since adding more than a trace of soft solders
might quickly have affected the ease with which you could make sheet
or wire, not to mention finished jewelry… Is it, for example,
possible that the original metal wasn’t sterling? Some old work might
have been 900 coin silver, or from some sources, even lower silver
content. Such metal, even if not contaminated might react differently
or more strongly for some people.